TWI496946B - Method for producing zinc-aluminum alloy-coated steel sheet with superior workability and corrosion resistance - Google Patents

Description

Method for manufacturing zinc-aluminum alloy plated steel sheet excellent in workability and corrosion resistance

The present invention relates to a method for producing a zinc-aluminum alloy plated steel sheet excellent in workability and corrosion resistance.

In order to ensure the corrosion resistance of the base steel sheet, the galvanized steel sheet is widely used based on economy and abundant resources, and is the most commonly used plated steel sheet. Further, many studies have been conducted to improve the corrosion resistance of galvanized steel sheets. In particular, an aluminum-plated steel sheet (also referred to as "Galvalume") having a 55% Al-Zn content was proposed in the late 1960s, and until now it exhibited excellent corrosion resistance and beautifulness. Exterior.

Compared with galvanized steel sheets, such aluminized steel sheets are superior in corrosion resistance and heat resistance, and thus are widely used in automobile mufflers, household appliances, heat-resistant materials, and the like.

For example, Japanese Laid-Open Patent Publication No. SHO-57-47861 discloses an aluminum-plated steel sheet containing Ti in iron, and Japanese Laid-Open Patent Publication No. SHO63-184043 discloses aluminum plating containing C, Si, Cu, Ni and a small amount of Cr in iron. An aluminum-plated steel sheet containing 0.01 to 4.0% of manganese, 0.001 to 1.5% of titanium, and 3.0 to 15.0% of niobium is disclosed in Japanese Laid-Open Patent Publication No. Hei 60-243258.

In addition, in order to suppress the growth of the Fe-Al alloy layer based on the reaction of aluminum and iron or to inhibit the rapid diffusion of aluminum metal into the iron, it is added to the aluminum plating bath. 10% or less of Si. Since the plated steel sheet produced by this method is excellent in workability and heat resistance, it is widely used as a heat-resistant member such as a muffler, a water heater, a heater, and an electric cooker liner of an automobile.

However, the addition of niobium in order to suppress the formation of the alloy layer may often impair the surface appearance of the plated steel sheet and disadvantageously make the surface appearance unclear. In this regard, it is known that this surface damage due to the addition of hydrazine can be solved to some extent by the addition of a small amount of magnesium (Sprowl, U.S. Patent No. 3,055,771).

Further, in recent years, the life of components for automobile exhaust systems has been extended, and the development of steel sheets obtained by introducing Cr into aluminum-plated steel sheets has been brought about. For example, JP-A-63-18043 discloses a plated steel sheet containing 1.8 to 3.0% of chromium, and a steel sheet containing 2 to 3% of chromium is disclosed in Japanese Laid-Open Patent Publication No. S63-47456.

At the same time, the Zn-Al alloy plated steel sheet exhibits the disadvantage that the processed cut portion cannot sufficiently exhibit corrosion resistance. This phenomenon is caused by a decrease in corrosion resistance due to a decrease in the sacrificial corrosion-resistant zinc which prevents the corrosion of iron from being exposed on the side of the cut portion of the zinc-aluminum alloy layer. In addition, the Zn-Al alloy plated steel sheet has a disadvantage of deterioration in corrosion resistance after processing because a plating layer having no heteroalloy phase is formed and the interface surface is easily damaged when used after bending or drawing, so The corrosion resistance after processing is deteriorated.

In order to solve such a phenomenon, Korean Patent No. 0586437 discloses a method of plating a Zn-Al-Mg-Si alloy plated steel sheet material excellent in corrosion resistance in a plating bath, the plating bath containing 45 to 70% by weight. Aluminum, 3 to 10% by weight of magnesium, 3 to 10% by weight of bismuth, the balance being zinc and unavoidable impurities, and A Zn-Al-Mg alloy plated steel sheet excellent in corrosion resistance and workability is disclosed in Korean Patent No. 0928804.

Accordingly, the present invention has been made in view of the above problems, and a primary object of the present invention is to provide a method of plating a zinc-aluminum-based alloy plated steel sheet by simultaneously adding it to a conventional zinc-aluminum-bismuth plating bath. Appropriate content of chromium, magnesium and titanium to further improve workability and corrosion resistance.

Another object of the present invention is to provide a method of plating a zinc-aluminum-based alloy plated steel sheet by suppressing the formation of a MgO oxide film on the surface of a plated steel sheet by adding a predetermined amount of calcium to a plating bath.

According to the present invention, the above and other objects can be attained by providing a method for producing a zinc-aluminum-based alloy plated steel sheet having excellent workability and corrosion resistance, which comprises containing 35 to 55 wt% of zinc, 0.5~. 3.0% by weight of ruthenium, 0.005 to 1.0% by weight of chromium, 0.01 to 3.0% by weight of magnesium, 0.001 to 0.1% by weight of titanium, balance of aluminum and unavoidable impurities in the plating bath to the base zinc-aluminum base The alloy plated steel sheet is subjected to a plating treatment to form a plating layer.

According to another aspect of the present invention, there is provided a method of producing a zinc-aluminum-based alloy plated steel sheet by further adding 1 to 10% by weight of calcium based on the total weight of magnesium to a plating bath to inhibit plating Formation of a MgO oxide film on the surface of the coated steel sheet.

The above plating layer can form Mg ₂ Si phase, Al phase, Zn phase, MgZn ₂ phase, Al-Zn-Mg-Ca mixed phase, Al-Zn-Si-Cr- on a typical surface structure of a common galvanized steel sheet. A combination of a Ca mixed phase and an AlCr ₂ phase.

At the same time, considering the deterioration of the corrosion resistance due to the low adhesion amount of the plating layer and the crystal grains which are unstablely grown therefrom, and the low economic efficiency due to the high adhesion amount, it is preferable to set the plating adhesion amount to 20 to 100 g/ m ² (based on a surface).

Further, the temperature of the plating bath is 550 to 650 ° C, and the cooling rate after plating is controlled to 15 to 30 ° C / sec.

According to the present invention, by appropriately adjusting the composition of the plating bath to control the size of the intermetallic compound layer and the crystal grains, it is possible to produce a zinc-aluminum alloy plated steel sheet excellent in workability and corrosion resistance.

Further, by adding an appropriate amount of calcium to the plating bath and surrounding the periphery of the plated steel sheet from the plating bath with a nitrogen cloud, the problem of suppressing the formation of the MgO oxide film on the surface of the plated steel sheet and the deterioration of the surface quality of the plating layer is solved. .

Therefore, the present invention is expected to be widely applied to fields requiring corrosion resistance, such as interior and exterior decorative materials for buildings, parts for home electric appliances, and heat resistant materials.

〔this invention〕

1‧‧‧ plated steel

10‧‧‧Exhaust nitrogen

2‧‧‧ air knife

3‧‧‧ plating bath

41, 42‧‧‧ lower nitrogen strip

43‧‧‧ side cover

44, 45‧‧‧ upper nitrogen strip

46‧‧‧Nitrogen supply tube

5‧‧‧ lifting device

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description of the accompanying drawings in which: FIG. 1 shows the surface of the galvanized zinc-based Mg and Mg-Cr-Ca plated steel sheets. Photo (1000x magnification).

Fig. 2 is a cross-sectional photograph (magnification 2000 times) of an aluminum-plated zinc-based Mg and a Mg-Cr-Ca plated steel sheet.

Fig. 3 is a view showing the formation of a film for suppressing the formation of MgO oxide by the addition of aluminum-zinc-based Mg and Mg-Cr-Ca.

Figure 4 is a diagram showing the plane of a device for forming a nitrogen dam (a device for forming a nitrogen cloud) schematic diagram.

Fig. 5 is a cross-sectional view taken along line A-A' of Fig. 4.

Figure 6 is a schematic illustration of the apparatus shown in Figure 4.

Hereinafter, the present invention will be described in more detail.

In the method of the present invention, the plating bath contains 35 to 55% by weight of zinc. Zinc has excellent sacrificial corrosion resistance to iron and thus acts to inhibit corrosion. It is necessary to obtain zinc by 35% by weight or more. This is because when the zinc content is lower than the range, the temperature of the plating bath will rise, thereby increasing dross and reducing workability, which has a negative effect on the operation. Further, when the zinc content is 55 wt% or more, the increase in the specific gravity of the plated steel sheet leads to an increase in production cost and a decrease in economic efficiency.

The plating bath of the present invention contains 0.5 to 3.0% by weight of cerium. The crucible has an action of suppressing the growth of the alloy layer, and can effectively increase the fluidity of the plating bath and impart a gloss effect, and should be added in an amount of 0.5% by weight or more. The key role of niobium in the plating layer is to control the formation of an alloy layer of the base steel sheet and aluminum. When the amount of ruthenium added is 0.5% by weight or less, the function of ruthenium is limited and the workability is remarkably lowered. On the other hand, if the amount of cerium added exceeds 3% by weight, Mg ₂ Si which is a factor contributing to the improvement of the corrosion resistance of the plating layer is excessively generated and grown on the surface of the plating layer, so that the surface is rough and early Coating characteristics that cause surface discoloration and hinder post-treatment. Therefore, the amount of rhodium added is preferably from 0.5 to 3% by weight.

The chromium added to the plating bath acts to form a dense passive oxide film on the surface of the plating layer, improving the corrosion resistance of the aluminum plated steel and because of the chromium The crystal grains of the plating layer are miniaturized by being uniformly distributed in the plating bath.

Further, the chromium functions as a band (Fig. 2) of a mixed phase of Al-Zn-Si-Cr which is formed in a predetermined form in the plating layer. The chromium present in the plating layer reacts with aluminum to form an AlCr ₂ phase, which serves to improve workability and corrosion resistance of the fracture surface after processing. The chromium makes it possible to control the content of the above ruthenium to 3% by weight or less and to prevent excessive precipitation of ruthenium in the needle-like form in the plating layer.

It is known that the content of chromium which provides such an effect is 0.1 or more (U.S. Patent No. 3,055,771 to Sprowl). However, the content of chromium in the method of the present invention is 0.005 to 1.0% by weight. If the content of chromium is 0.005% by weight or less, chromium cannot be rapidly and uniformly distributed in the plating bath, and when the content is 1.0% by weight or more, the temperature of the plating tank needs to rise as the chromium content increases, and the scum (dross) increases, and the scum adheres to the surface of the plated steel sheet to impair the appearance.

Therefore, the amount of chromium added is preferably from 0.005 to 1.0% by weight.

Further, the plating bath of the present invention contains 0.01 to 3.0% by weight of magnesium.

The magnesium added together with the chromium combines with the oxygen in the air contacting the plating layer to form a passive film to prevent diffusion of oxygen into the interior of the alloy layer and to suppress additional corrosion to improve corrosion resistance. In the process of etching and forming a local battery, the Mg ₂ Si phase formed by the reaction of magnesium and cerium in the plating layer (refer to Figures 1 and 2) and the MgZn ₂ phase formed by the reaction of magnesium and zinc, Through the sacrificial corrosion resistance of zinc, it plays a role in reducing the corrosion rate. In addition, magnesium reacts with aluminum to block the penetration of oxygen, thereby significantly improving the corrosion resistance of the sheared section.

If the amount of magnesium added is 0.01% by weight or less, the degree of dispersion and When the corrosion resistance is less than 3.0% by weight, the plating bath is saturated, the melting point is increased, the workability is lowered, the surface quality is lowered due to the continuous occurrence of dross on the surface, the production cost is increased, and the production process is increased. Related issues have become serious.

Therefore, the amount of magnesium added is preferably from 0.01 to 3.0% by weight.

The plating bath of the present invention further contains 1 to 10% by weight of calcium relative to the weight of magnesium. Calcium added together with magnesium and chromium prevents the formation of magnesium oxide on the interface of the molten metal to be plated, and thereby passes through the fine magnesium oxide film adhering to the surface of the plated steel sheet to prevent deterioration of appearance quality.

It is known that the addition of Ca, Be, Al or the like to a molten Mg bath remarkably suppresses oxidation and combustion of the molten Mg bath even at a high temperature. The mechanism for suppressing the oxidation of the molten Mg bath by adding calcium is to increase the combustion temperature of the molten Mg bath to 200 ° C or more by adding calcium. The increase in the combustion temperature of such a Mg alloy generally causes the oxide layer usually formed on the surface to change from a porous oxide layer to a dense oxide layer, whereby the oxygen permeation can be effectively blocked.

When the amount of calcium added is 1% by weight or less based on the weight of magnesium, the degree of dispersion is deteriorated and the effect of suppressing the MgO oxide film is lowered, and if it exceeds 10% by weight based on the weight of magnesium, there is a possibility that aluminum and calcium are formed. The coating layer due to the intermetallic compound has low processability. Therefore, the amount of calcium added is preferably from 1 to 10% by weight based on the weight of the magnesium.

The present invention applies a nitrogen nozzle connection dam (Dam) capable of nitrogen purging and preventing adsorption of an oxide film to a strip to the lower surface of the air knife of the plating bath. After the test piece is immersed in the plating bath, the lower surface of the air knife which rises up to the interface of the plating bath is purged in a nitrogen atmosphere, thereby suppressing the growth of the oxide film. In order to prevent the fine oxide film generated after contact with the air on the outer side of the surface of the molten plating bath from flowing into the dam and adsorbing on the test piece, nitrogen wiping is performed on the lower surface of the nitrogen dam through the nitrogen curtain nozzle (Wiping) ).

Further, the plating bath of the present invention further contains 0.001 to 0.1% by weight of titanium in order to reduce the size of the spangle which constitutes the appearance of the plating layer and forms a pattern of the plating layer. When the amount of titanium added is 0.001% by weight or less, the degree of dispersion on the steel sheet is lowered, and if it is 0.1% by weight or more, it is difficult to dissolve in the plating bath, and titanium cannot improve the effect.

The invention is based on the reduction of the size of the sequins by adding a suitable amount of chromium, magnesium, calcium and titanium to the plating bath containing aluminum, zinc and bismuth to increase the possibility of nucleation on the conventional galvanized steel plate. To achieve.

That is, after the steel sheet is plated, the added components are dispersed in the plating layer to form various nuclei of the Mg ₂ Si phase, the MgZn ₂ phase, and the AlCr ₂ , and the mutual interference between the grain boundaries controls the crystal grains. Growing.

Therefore, a beautiful surface appearance is ensured, corrosion between grain boundaries is suppressed, and corrosion resistance is enhanced. Further, the growth of the alloy layer of aluminum and iron is suppressed and a plating film excellent in workability is thereby formed.

At the same time, it is preferable to set the temperature of the base steel sheet immersed in the molten plating solution to 570 to 650 ° C, and to set the temperature of the molten plating bath to 550 to 650 ° C.

This is because if the bath temperature of the base steel sheet is lower than 550 ° C, the fluidity of the plating bath is lowered, the appearance of the plating film is poor, and the adhesion of the plating film is lowered. On the other hand, if the bathing temperature is 650 ° C or higher, the rapid thermal diffusion of the base steel sheet causes abnormal growth of the alloy layer, so that the workability is lowered and melted. Excessive Fe oxide layer is formed in the bath.

The plating amount is preferably from 20 to 100 g/m ^{2 on the} basis of one side. When the plating amount is 20 g/m ² or less, the air pressure of the air knife device for controlling the plating amount is excessively increased, and variations in plating amount occur, and the appearance of the film is damaged by the rapid increase of the surface oxide in the molten plating bath. And the attachment of oxide scum.

Further, when the plating amount is 100 g/m ² or more, the alloy layer is excessively formed, and the workability is remarkably lowered.

Meanwhile, the present invention provides an apparatus for forming a nitrogen film which permeates the periphery of a plated steel sheet which is upward from the surface of the plating bath with nitrogen gas to further suppress formation of an oxide film on the surface of the plated steel sheet.

Figures 4 through 6 are schematic views of the apparatus of the present invention.

The apparatus of the present invention is spaced apart from the surface of the plating bath 3 by a predetermined distance and is moved up and down between the surface of the plating bath 3 and the air knife 2 through the lifting device 5.

The apparatus of the present invention includes lower end nitrogen discharge strips 41 and 42 formed in a square shape around the plated steel sheet 1 upward from the surface of the plating bath 3. The lower end discharge bars 41 and 42 receive nitrogen from the nitrogen supply pipe 46 disposed at the side thereof and discharge nitrogen gas toward the surface of the plating bath 3. Although not shown in the drawings, a plurality of holes (nozzles) for ejecting nitrogen gas are disposed at predetermined intervals on the lower surfaces of the lower end nitrogen discharge bars 41 and 42.

The lower end nitrogen discharge strips 41 and 42 are square tubes which can be formed in an integral manner. As shown in Fig. 4, the first strip 41 and the second strip 42 are branched and separated from each other, thereby being spaced apart from each other in the width direction (in the longitudinal direction in the drawing).

Further, the apparatus of the present invention includes a side cover 43 which is inclined upward at an angle, extending from the side of the lower end nitrogen discharge strips 41 and 42 toward the plated steel sheet 1; and the upper end nitrogen discharge strips 44 and 45, Nitrogen gas 10 is formed on the upper surface of the side cover 43 and downward.

The upper end nitrogen discharge strips 44 and 45 are tubes having nitrogen discharge holes (not shown) facing the surface of the plating bath, and the tubes face each other on the upper surface of the side cover 43, and discharge nitrogen gas to the inside. The upper nitrogen strips 44 and 45 receive nitrogen from the nitrogen supply tube 46.

At the same time, the side cover 43 extends upward from the lower end nitrogen discharge bars 41 and 42 to the upper end nitrogen discharge bars 44 and 45 toward the plated steel sheet 1 at an angle upward. Thereby, the discharged nitrogen gas 10 can stay on the periphery of the plated steel sheet 1 without being diffused.

By using the apparatus for forming a nitrogen cloud according to the present invention, by forming a nitrogen cloud 47 on the periphery of the relatively high-temperature plated steel sheet 1 rising from the surface of the plating bath 3, it is possible to suppress the oxide film on the surface of the plating bath 3 described above. form.

Hereinafter, the present invention will be described in more detail by comparison between the examples and the comparative examples. The examples are provided to describe the invention in more detail and are not to be construed as limiting the scope of the invention.

The cold-rolled steel sheet having a thickness of 0.8 mm, a width of 120 mm, and a length of 250 mm was plated by a melt plating simulator. As shown in Table 1, a zinc-aluminum-based alloy plated steel sheet was produced by changing the composition of the plating bath. Further, a nitrogen cloud was formed using the nitrogen cloud forming apparatus shown in Figs. 4 to 6.

The plating amount was controlled by an air knife, and the evaluation results based on the plating amount of one side of the produced zinc-aluminum-based alloy plated steel sheet are shown in Table 1.

The evaluation items are corrosion resistance and workability. Corrosion resistance was compared and evaluated according to the KSD 9504 test method in an initial rust generation time (5%) under a sodium chloride (NaCl) salt spray test atmosphere of 5% and 35 °C. Workability According to the KSD 0006 test method, the width (the width of the crack) generated after the 180° OT bending test was observed by a stereo microscope of 30 to 50 magnification. The width of the fracture surface was measured to compare and evaluate. X-ray diffraction (XRD) was used to observe the alloy phase.

The detailed test results obtained by the test method are as follows.

1. Machinability: observed crack width level

◎: The fracture width is 10 to 20 μm.

△: The fracture width was 20 to 30 μm.

X: The fracture width is 40 to 50 μm.

2. Scum level: the amount of scum generated in the upper portion of the plating bath after the plating sample of the plating composition according to the present invention is melted

◎: The amount of scum generated was 5% or less with respect to the plating bath.

△: The amount of dross generated is 10% to less than 20% with respect to the plating bath.

X: The amount of scum generated was 20% or more with respect to the plating bath.

3. Surface appearance: the clarity and formation level of the sequins of the surface appearance of the plating layer observed by the naked eye

◎: The formation of the sequins is clear and the gloss is high.

△: The formation of the sequins is not clear.

X: The formation of the sequins is small and the appearance is poor.

4. Corrosion resistance of the cut surface: Corrosion ratio generated after 1000 hours of salt spray test

◎: The iron corrosion ratio is 5% or less.

△: The iron corrosion ratio is 10 to 20%.

X: The iron corrosion ratio is 30% or more.

5. Corrosion resistance of flat portion: Corrosion ratio generated after 2500 hours of salt spray test

◎: The iron corrosion ratio is 5% or less.

△: The iron corrosion ratio is 20 to 30%.

X: The iron corrosion ratio is 30% or more.

As shown in Table 1, the workability and corrosion resistance of the examples of the present invention were excellent. That is, in the examples of the present invention, since the crack (fracture surface) generated after the 180° OT bending test was performed was about 10 to 20 μm, the corrosion resistance was better than that of the comparative example. In the embodiment of the present invention, the plating amount is 50 g/m ² on one side, and the total cross-section rust is generated after 3000 hours or more, and rust is generated on the cross-section after 1000 hours or more. This result shows that the embodiment of the present invention exhibits more excellent corrosion resistance than the conventional composition.

Further, as a result of visual observation, the examples exhibited a more excellent surface appearance than the comparative examples. This is due to the miniaturization of the size of the sequins.

While the preferred embodiment of the present invention has been disclosed for the purpose of illustration, it will be understood by those skilled in the art that various changes, additions and substitutions can be made without departing from the scope and spirit of the invention disclosed in the appended claims. .

1‧‧‧ plated steel

3‧‧‧ plating bath

41, 42‧‧‧ lower nitrogen strip

43‧‧‧ side cover

44, 45‧‧‧ upper nitrogen strip

46‧‧‧Nitrogen supply tube

5‧‧‧ lifting device

Claims (2)

A method for producing a zinc-aluminum-based alloy plated steel sheet excellent in workability and corrosion resistance, which comprises containing 35 to 55% by weight of zinc, 0.5 to 3% by weight of bismuth, 0.005 to 1.0% by weight of chromium, The base zinc-aluminum alloy plated steel sheet is plated to form a plating layer in a plating bath of 0.01 to 3.0% by weight of magnesium, 0.001 to 0.1% by weight of titanium, and the balance of aluminum and unavoidable impurities. The plating layer forms a Mg ₂ Si phase, an Al phase, a Zn phase, a MgZn ₂ phase, an Al-Zn-Mg-Ca mixed phase, and an Al-Zn-Si- on a typical surface structure of a common aluminum-zinc-plated steel sheet. A combination of a Cr-Ca mixed phase and an AlCr ₂ phase. The method of claim 1, wherein the plating bath further comprises 1 to 10% by weight of calcium based on the total weight of the magnesium.